1. Field of the Invention
The present invention relates to a wavelength-division multiplexing optical communication method suitable for use in local area networks (LAN), or the like, and an optical communication system therefor.
2. Description of the Related Art
In conventional wavelength-division multiplexing optical communication methods, light signals having different wavelengths are transmitted over a single transmission line to enhance the efficiency of the transmission line. Conventional wavelength-division multiplexing optical communication systems generally include a plurality of terminals for transmitting light signals having different wavelengths, a light superimposition means for supplying the light signals to a single transmission line, a single transmission line, a separation means for separating required wavelength components from a wavelength-division multiplex signal, and a plurality of terminals for receiving the separated signals.
A semiconductor laser is generally used as a light source for emitting light from each terminal. An optical fiber is often used as the transmission line. The light superimposition means for supplying light signals having a plurality of wavelengths to the single optical fiber may be a light combining element, such as a half mirror or a beam splitter, or a light combining element which employs an optical wave-guide.
To separate a required wavelength component from the wavelength-division multiplex signal, a light wavelength filter or a prism is conventionally used.
Alternatively, the light heterodyne method is used to separate a required wavelength component. In the light heterodyne method, a light having a wavelength very close to that of a desired signal is mixed with the wavelength-division multiplex signal, and the resultant signal is converted into an electrical signal by means of a photo-detector. The obtained electrical signal is electrically filtered to obtain a difference frequency signal which is the required signal. The light heterodyne method has been attracting attention, because it is very effective to enhance the degree of wavelength-division multiplexing.
As stated above, optical communications generally employ, as a light source, a semiconductor laser which emits light whose wavelength is readily varied by the ambient temperature or other factors, and thus have the following drawbacks:
(1) In the method in which a required wavelength component is separated from the wavelength-division multiplex light signal using a light wavelength filter or a prism, the wavelength bandwidth for each light signal must be sufficiently broad in order to prevent interference of light signals having various wavelengths at the reception side during the separation which occurs when the wavelength of the light emitted from the semiconductor laser varies. This widens the range of the wavelengths employed in the communication, and thus, hinders an increase in the degree of multiplexing.
(2) In the method of separating the required wavelength component using the light heterodyne detection method, the wavelength of the light must be firmly fixed. To achieve this, the temperature of the light source portion is strictly controlled. In a case when very high wavelength stability is required, feedback control is performed by monitoring the wavelength of the light emitted by means of a spectrometer, or the like, and by using that wavelength to manipulate an input wavelength so as to bring the value of the controlled wavelength closer to a desired value.
However, any of these controls requires large-scaled circuits, thus increasing the production cost. Therefore, such systems are used only for particular optical communication systems, such as ones conducted over main lines.